This invention relates generally to fluid processing, and specifically to diagnostic techniques for flow control systems. The diagnostic techniques are directed toward flow obstructions, corrosion, wear and tear on electromechanical valve components, and other forms of electromechanical deterioration or flow control impairments.
Safe, accurate, and cost-effective flow control is critical to a wide range of industrial, engineering, and scientific processes. Mass-based flow control systems have substantial utility in these areas, particularly in applications such as semiconductor fabrication and pharmaceutical manufacturing, where precise and absolute flow control is required. A primary advantage of mass-based flow control systems is that they operate substantially independently of pressure and temperature effects, making them less susceptible to environmental bias and related systematic concerns that characterize other, more traditional flow control technologies.
A typical mass flow control system includes an upstream thermal mass flow meter and a downstream valve controller. The thermal mass flow meter comprises a heat source and (usually two) temperature sensors, such as thermocouples or resistance-temperature devices (RTDs), arranged along a sensor tube. The valve controller typically includes a valve coil assembly or similar structure, configured to control the process flow by positioning or actuating a valve.
The heat source and the temperature sensors are thermally coupled to the sensor tube, which is typically a bypass flow tube running parallel to a main process flow. The heat source imparts thermal energy to the fluid in the sensor tube, creating a differential signal across the temperature sensors. The flow through the sensor tube is a function of the temperature differential, and the total process flow is a function of the flow through the sensor tube. Thus the temperature differential signal characterizes the total process flow rate. The process flow is controlled by the valve coil assembly, which positions a valve plunger downstream of the flow sensor.
A functional relationship between the mass flow rate and the temperature differential is determined by the conservation of thermal energy and the geometry of the flow sensor. This relationship is substantially independent of the fluid's pressure and temperature, providing for precise and absolutely calibrated control of process fluid flow. In particular, mass-based flow control systems are inherently less sensitive to environmental bias and other systematic effects than volumetric, differential pressure, or velocity-based systems.
Mass flow controllers are, however, susceptible to certain operational impairments related to their precision measurement and control mechanisms. These include flow obstructions and corrosion, such as in the sensor tube, and deterioration of electromechanical components, particularly in the valve controller. Mass flow control systems thus require regular maintenance, which is traditionally based on service hours, number of valve cycles or excursions, and other life expectancy-related measures.
Unfortunately these measures are indirect, rather than direct, indicators of the system's actual operational condition. While service hours and valve cycles may be statistically associated with operational impairments, that is, they provide little direct indication of any particular device's actual physical condition. This forces a tradeoff between maintenance costs and failure risk, as expressed in an idealized service life expectancy.
In order to capture devices that actually suffer operational impairments, this tradeoff necessarily requires the regular replacement of other, fully serviceable components as well. This is characteristic not only of service hour and valve cycle-based maintenance programs, but of any diagnostic technique that relies on indirect, rather than direct, operational indicators. Indirect techniques may also fail to address the reality of widely variable operating conditions, which can significantly affect failure probabilities.
There is thus a need for flow diagnostic techniques that are more directly indicative of actual operational conditions, and which can facilitate efficient, cost-effective maintenance programs. The techniques should be directly indicative of specific operational conditions such as flow obstructions, corrosion, wear and tear, electromechanical deterioration, and other operational impairments, and should be independent of indirect indicators based on service life expectancy. The diagnostics techniques should also be applicable on an individual basis, without reference to other process devices, and should be easily integrated into existing process control systems without the need for substantial modifications.